Many early immune responses to damage or infection are initiated by a family of innate immune toll-like receptors (TLRs). The TLR family has broad specificity, recognizing molecular structures that are highly conserved among pathogenic organisms or different types of physical damage. The ligands of specific TLRs include endotoxin, single- or double-stranded RNA, peptidoglycan, flagellin, heat shock proteins, and in the case of TLR9, unmethylated CpG sequences in DNA (Bell et al., Trends Immunol. 24:528-533 (2003) and Wagner, Trends Immunol. 25:381-6 (2004)). While this early sensing of disease or damage is critical to the ensuing immune response, it can also be the source of inappropriate or damaging responses as well. For example, TLR9 has been implicated in autoimmune disorders, including lupus, which has long been associated with anti-DNA antibody reactivity, and multiple sclerosis (Prinz et al., J Clin Invest 116:456-464 (2006)). TLR9 also appears to underlie the uncontrolled inflammatory response that is associated with death from septic shock (Plitas et al., J Exp Med 205: 1277-83 (2008)).
TLR9 was first identified as an innate immune receptor expressed by human B cells and plasmacytoid dendritic cells (PDC) that binds unmethylated CpG sequences, resulting, in cellular activation and cytokine secretion (Hemmi et al., Nature 408:740-745 (2000)). These sequences are over-represented in bacterial DNA in comparison to eukaryotic DNA, and therefore can serve as an indicator of bacterial infection. However, eukaryotic DNA contains some unmethylated CpG and is capable of stimulating TLR9 as well (Vallin et al., J Immunol. 163:6306-13 (1999) and Leadbetter et al. (2002)). While DNA from dying cells is typically ingested and sequestered from the system, lupus is associated with genetic defects that lead to poor clearance and accumulation of excessive cell debris (Krislman et al., Seminars in Immunology 18:240-243 (2006)), exposing, the immune system to abnormally high levels of ligand. This condition, combined with targeted uptake of DNA-containing complexes by either complex-specific immunoglobulin on B cells, or Fc receptors on dendritic or antigen-presenting cells, can result in presentation and reaction to autoantigen, a response boosted by the presence of TLR9 ligand in the stimulating complex. This observation is exemplified by the fact that PDC respond to complexes found in systemic lupus erythematosus (SLE) serum by secreting α-interferon, and that this is Fc receptor and DNA-dependent and mediated by TLR9 (Leadbetter et al., (2002) and Means et al., J Clin Invest. 1152:407-17 (2005)). The resulting α-interferon can further drive dendritic cell and B cell maturation. Correspondingly, array data show activation of α-interferon inducible genes in patients with severe disease (Bennett et al., J Exp Med. 197:711-23 (2003)). As PDC are the predominant source of α-interferon in the body, this observation further associates this cell population with disease.
B cells are also central to lupus autoimmunity, and are driven to proliferate and produce IL-6 and antibodies by TLR9 stimulation with synthetic oligonucleotides. Due to preferential association with B cells expressing autoreactive cell surface immunoglobulin, autoantigen-specific cells will predominate in this response. A self-reinforcing cycle is thus at work, in which DNA or RNA-containing cell debris combines with antibodies from TLR9-stimulated autoantigen-specific B cells to stimulate plasmacytoid dendritic cells via Fc receptors.